1. Field of the Invention
The present invention is directed to an apparatus for electrochemically determining the partial oxygen pressure pO.sub.2 of a liquid measuring medium, such as blood, including a measuring sensor provided with a working electrode and a counter-electrode for chemically reducing the oxygen (O.sub.2) and having a control means for controlling the potential of the electrodes.
2. Description of the Prior Art
Measuring arrangements for electrochemically determining the partial oxygen pressure pO.sub.2 in the blood of a patient are known, and are used for patient monitoring. Knowledge of the partial oxygen pressure, for example, is a necessary condition for assessing the respiratory condition of an artificially respirated patient. The polarographic measuring method (reduction of oxygen) is used as the standard method for measuring the partial oxygen pressure pO.sub.2. This oxygen reduction is essentially defined by the electrode material which is used, and by the pre-treatment and composition of a measuring electrolyte. A cathode, which may consist of platinum, is provided as a working electrode, and an Ag/AgCl anode is provided which serves as a counterelectrode. The current through the platinum cathode, upon the application of a defined potential, is a function of the partial oxygen pressure pO.sub.2 in the measuring medium. Deposits from the measuring medium on the surface of the platinum cathode may, however, lead to a poisoning of this electrode. A constant recalibration of the electrode system is therefore required.
The problem of electrode poisoning is partially solved in a known arrangement described in "Medical and Biological Applications of Electrochemical Devices", Koryta (1980) at pages 178-179. In this known arrangement, the electrode system contains a platinum cathode as the working electrode and an Ag/AgCl anode, which are provided with an electrolyte and which are covered with a hydrophobic membrane. The membrane allows a gas exchange between the measuring medium and the electrolyte, but prevents the penetration of substances which could cause a poisoning of the electrodes. Because additional substances are required in the electrolyte, given the reduction of oxygen necessary for the measurement, or arise as a result of the measurement, an adequate supply of this electrolyte must be provided. This known arrangement is thus not suitable for miniaturized measuring cells, as are needed for patient monitoring. Moreover, a relatively long time elapses until equilibrium is established between the electrolyte and the measuring medium, thereby preventing rapid measurement of variable partial oxygen pressures pO.sub.2.
Another known apparatus is described in U.S. Pat. No. 4,853,091. In this arrangement, an oxygen sensor is provided which has an uncovered electrode, which may consist of precious metal. The working electrode, for example, may consist of gold and the counter-electrode may consist of platinum. In order to prevent poisoning of the working electrode, two different potentials are successively supplied to this electrode, the successive application of the different potentials being referred to as a cycle. Only a portion of the cycle duration is used as a measuring phase, the measuring phase being short compared to the total cycle duration. The cycle thus consists of a relatively short measuring phase and a relatively long regeneration phase. The charge converted at the electrodes serves as measured signal. Evaluation of the measured signal begins with a chronological delay of at least a few milliseconds following the beginning of the measuring phase. In addition to the working electrode and the counter-electrode, this oxygen sensor can be provided with a reference electrode which may consist of silver/silver chloride. The surface of the electrodes is thus in contact with the measuring medium both during the measuring phase and during the regeneration phase.